In recent years, there has been much interest in producing a variety of different molded parts by the so-called "reaction injection molding" (RIM) process. This process involves a technique of filling a closed mold with highly reactive, liquid starting components within a very short time, generally by using high output, high pressure dosing apparatus after the components have been mixed in so-called "positively controlled" mixheads.
The RIM process is widely known and a detailed description of the technology thereof may be found, for example, in the following references:
Piechota/Rohr: "Integralschaumstoffe", Carl Hanser-Verlag, Munich/Vienna, 1975; PA1 Prepelka/Wharton: "Reaction Injection Molding in the Automotive Industry", Journal of Cell. Plastics, Vol. II, Nov. 2, 1975 PA1 Knipp: "Plastics for Automobile Safety Bumpers", Journal of Cell. Plastics, Nov. 2, 1973. PA1 Ludwico/Taylor: "The Bayflex 110 Systems--The New Generation of RIM Materials", presented at SAE Passenger Car MTG, Detroit, September 1977.
The reaction injection molding technique may be used for producing large moldings weighing from 3 to 10 kg or more, such as the flexible car body parts also known in the motor industry as "soft face elements", i.e. reversibly shaped front and rear parts of motor vehicles.
The following technical advance is generally achieved by the RIM procedure: large quantities of liquid, highly reactive starting materials are delivered mechanically within a very short time (from about 2 to 4 seconds), are mixed, and are introduced into a closed mold in which the mixture is cured to yield the finished product within a time (from 30 to 120 seconds) which is also very short for polyurethane materials.
Realization of this technology required a solution to the following three problems:
1. In view of the high reactivity of the starting components (polyisocyanates and compounds which are reactive with polyisocyanates) the reaction mixture must be introduced into the mold within the shortest possible time which should not exceed the cream time (i.e., the time between mixing of the reactants and the first visible signs of a chemical reaction). This necessitated the development of highly efficient axial and radial piston pumps which when installed in high pressure machines were capable of delivering at a rate of from 2.5 to 6.5 kg/second. Machines of this type are known and have been described, for example, in German Offenlegungsschriften Nos. 1,778,060 and 2,146,054.
2. Production of a faultless molding required not only exact dosing of the components to keep them at a particular ratio prescribed by the given formulation over the whole period of injection, but also required intimate mixing of the components from the first to last drop. Perfect mixing is made enormously difficult by the fact that due to their high flow velocities, the components have only a very short residence time in the mixing chamber of the mixing head. This problem could be solved by using so-called positively controlled "mixheads" which operate on the principle of counterflow injection (see e.g., U.S. Pat. Nos. 3,709,640 and 3,857,550, and German Offenlegungsschriften Nos. 2,007,935 and 2,364,501).
3. When the reaction mixture enters the closed mold, it almost instantly displaces the air contained in the mold. To ensure that this does not lead to inclusions of air in the reaction mixture and hence faults in the end product, the liquid streaming into the mold must, in effect, "push" the air forward in front of it in the form of a "flow front" and expel it through predetermined slots. To ensure complete absence of turbulence during filling of the mold, the material must enter the mold over a wide front along the wall of the mold in a laminar stream. This has been achieved by developing a certain technique of injection through so-called "film gates" described in German Offenlegungscchriften Nos. 2,348,658 and 2,348,608.
In spite of the many advantages of the RIM process, there is a continual searching for faster reactive systems, particularly for use in mass production industries, such as the automotive industry.
A very promising reactive system has recently been developed which is based on the use of an active aromatic diamine having at least one alkyl substituent in the ortho-position to a first amino group and two alkyl substituents in ortho-position to a second amino group. These active aromatic diamines are generally used in combination with organic polyisocyanates, hydroxyl group-containing materials and catalysts for the reaction between hydroxyl groups and isocyanate groups. These systems are the subject of U.S. Application Ser. No. 894,227 filed Apr. 7, 1978 now abandoned, a continuation of CIP Application Ser. No. 803,014 filed June 24, 1977, now abandoned which was in turn a continuation of Application U.S. Ser. No. 761,166 filed Jan. 21, 1977, now abandoned.
These active systems gel so quickly upon mixing of the components that the final products produced therefrom have relatively high densities (i.e., 70 lbs. per cubic ft.), since conventional organic blowing agents do not volatilize quickly enough to have any blowing effect. Consequently, the molds used are generally overpacked in order to ensure adequate filling of the mold. This overpacking necessarily results in (a) an increase in the pressure in the mold requiring increased clamping pressures to keep the mold closed and (b) visual surface defects. While the surface defects are not physical (i.e., they do not affect the surface physically and do not appear to affect the physical properties), their visual appearance renders them unacceptable for use in a variety of different applications, particularly for exposed automotive parts.
It would be desirable to reduce the density of the products of the above-noted reactive systems (e.g., to about 60 lbs. per cubic foot), while at the same time reducing the internal mold pressure and eliminating the surface defects.
Since conventional organic blowing agents are ineffective, one suggestion, which has met with some success, has been to include in one or more of the components air and/or nitrogen under pressure. The use of air and/or nitrogen in polyurethane systems is, of course, known, as are the many and varied techniques for providing such dissolved air and/or nitrogen. For example, air and/or nitrogen has been introduced directly into the mixing chamber and mixed simultaneously with the reactive mixture. Additionally, the air and/or nitrogen has been whipped into one or more of the components. The creamy mixture formed is then metered by means of a pump to a final mixing chamber where it is mixed with the other reactive components. When the metering pump discharges at a sufficiently high pressure, the quantity of gas which is initially dissolved and/or dispersed in the starting material, which is fed to the metering pump, dissolves at the higher pressure in a very short period of time. The liquid fed to the mixhead then contains gas in the dissolved state. Upon being fed to the mixhead, dissolution takes place in a very short time. In general, it is preferred that the gas be dissolved in one or more of the components. Other techniques for dissolving gases are also known and are described in e.g. U.S. Application Ser. Nos. 712,457, now abandoned, and 724,132 and now U.S. Pat. No. 4,089,206 filed on Aug. 6, 1976 and Sept. 17, 1976, respectively and in U.S. Pat. No. 4,050,896.
Although the use of such dissolved air and/or nitrogen has met with some success with the highly active systems noted above, it has been found that the resultant molded part, while of reduced density (e.g., from 62 to 68 lbs. per cubic ft.), will have varied densities throughout the molded part.
As noted above, air and/or nitrogen is effectively dissolved under pressure in one or more of the components. It has been observed that when this pressure is relieved (e.g., upon passage of the components through the mixhead and into the mold), the air and/or nitrogen does not immediately pass from the dissolved state to the dispersed state. It is believed that a state of super saturation exists in liquid reacting system containing the dissolved gas for some finite period of time. For these highly reactive systems, this delay in passage from the dissolved to the dispersed state is sufficiently long so that gelation occurs in the mold before proper blowing.
All of the above-noted problems have now been substantially removed by the instant invention.